1. Field of the Invention
The present invention relates to a light-metal-part blank which is to be cast into another light-metal casting, and has a roughness of more than 20 .mu.m on its outer surface, and also to a method for producing the blank in which method the surface of a blank is blasted with a directed jet of particles which consist of a hard material and are carried along in a flowing gas.
2. Description of the Prior Art
DE 44 38 550 A1 describes the casting of a cylinder liner into a crankcase. Casting separately manufactured cylinder liners into light-metal crankcases has successfully optimized the running properties of the reciprocating piston in the cylinder liner, irrespective of the material of the crankcase. Problems with casting the cylinder liners into the light-metal crankcase arise, however, due to the inadequacy of the bonding of the outside of the liner with the crankcase material. When the engine is running, materially imperfect bonding can cause the emission of waste heat from the reciprocating-piston engine to be impeded. In particularly unfavorable instances, this emission can even lead to a loosening of the cylinder liner in the crankcase. As regards other parts to be cast in, for example forged rotor recesses in a cast piston, good bonding is indispensable, for strength reasons alone.
DE 43 28 619 C2 discusses problems involved in good material bonding of the light-metal components during casting in, in particular in the instance of a cylinder liner to be cast in. An objective is a pore-free material union between the outside of the liner and the case material by controlled preheating of the cylinder liner. The cylinder-liner blank preheated to a specific temperature, for example 450.degree. C., and introduced into the casting mold has its surface melted (incipiently) by the inflowing melt of the case material, and an intimate bond with the case material is thereby made. A high melt flow directed parallel to the contact surface further assists this effect, not only by bringing about increased incipient melting as a result of a better heat exchange, but also by washing off the oxide skin, which is always present, from the contact side of the liner.
Such an intensive relative flow of the melt can be ensured by various measures. The above-mentioned publication mentions, for example, a choice and distribution of the gates, an agitation of the melt or even an induction of electrical eddy currents which cause fluid flows in the melt. A disadvantage of this method, however, is that the liner blanks preheated to temperatures which bring about reliable incipient melting are difficult to handle, especially during the casting of multi-cylinder crankcases. With the gradual introduction of the individual preheated liners into the casting die, either different liner temperatures have to be allowed for, due to cooling, during the casting operation or heating elements have to be provided in the casting die so that the liner blanks already introduced are kept hot, thus making the casting die more complicated and adversely affecting the dissipation of heat from the solidifying cast workpiece.
In any event, a preheating furnace must be installed, and this installation incurs further investment costs, above all, regular power-supply costs. Moreover, the high preheating temperatures may lead to undesirable structural changes in the material of the cylinder liner which can adversely influence the liner's running properties. Tribologically relevant structural changes are obtained if the liner blank, while being cast in, is melted down nearly into the region of the running surface.
A machining oversize of at least about 1 mm provided on the inside of the liner blank must be taken into account. In order, therefore, to prevent the liner blank from actually melting through at all locations, a correspondingly thick-walled blank has to be provided. For reasons of the smallest possible cylinder spacing, however, the cylinder liner should be as thin-walled as possible. If, for whatever reason, the liner is not sufficiently preheated, i.e. by way of precaution or through carelessness, then, at least in die casting, only very short periods of time are available for filling the mold and until solidification commences. Consequently, the aforementioned incipient-melting measures cannot take effect, or can take effect only very incompletely, in the short time periods available.